3-spool engines tend to be a tad quieter than 2-spool designs - for the simple reason that each shaft is better rotational speed optimised. This is particularly valid for high thrust settings, ie take-off. The RR Trent / RB211 are the only triple spool motors out there, GE and PW are 2-spool. On the RR powered A330 the fully enclosed rear cowl is claimed to also contribute to lower noise as the louder combustion exhaust gases are somehow "shrouded" by the outer bypass air flow and exit the engine as one.

The A330 (and 340) also have considerably better cabin soundproofing than the 777 does. When the original A350 was offered (pre XWB), Airbus looked at reducing the thickness of the interior to exterior cabin wall of the existing A330 to give a few more inches extra cabin width. This would have been pretty much entirely at the expense of soundproofing.